The invention pertains to a method and apparatus for spreading monomer mixtures in molds for the centrifugal casting of contact lenses.
In the monomer casting of contact lenses in rotating molds provided with a limiting sharp edge according to the Czechoslovak Pat. No. 116,260 (counterpart to U.S. Pat. No. 3,408,429), the monomer mixture has to be introduced as high as very near to the limiting edge. This has been achieved, e.g., by rotating the mold, before it is exposed to the polymerization conditions, to a higher speed than is the rotation speed eventually required for the formation of a precise meniscus just before starting the polymerization. Another method has consisted in introducing a fine stationary string into the rotating mold which stirs and lifts the rotating mixture up to the sharp edge. A similar effect of even spreading of the monomer mixture was also obtained by introducing a sharp stream of nitrogen into the rotating mold to stir the monomer mixture.
The above said methods cannot be used in the novel method of centrifugal casting in rotating columns, which carry a column of molds with monomer mixture, e.g., according to the Czechoslovak Pat. No. 159,359 and the Czechoslovak Patent Application No. PV 1027-83 now Czechoslovak Certificate of Authorship No. 237,592. It was possible to help the mixture in spreading in the molds which form a column in tubular magazines by subjecting the magazines to a very slow rotation in an oblique position, when the monomer mixture coalesced in a drop, which slowly moved along the limiting sharp edge. This method was fairly successful in the production of a relatively thick-walled lenses. As soon as the trend in contact lenses has been directed to ultrathin lenses, complications occured caused by the fact, that a thin film of a small charge of monomer mixture did not manage to flow as far as to the edge at a small slope of the slowly rotating mold or a rather tin film was formed in the center of mold at a too large slope of the rotating mold, which broke through thus forming a hole in the center of a nonwetted pair of the mold during the following polymerization.